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I am learning the new to me Sonerai II powered by an Aerovee 2.0. I have a lot to learn being a first time VW aero engine owner. Here are my problems from last weekend:

1. I did about 20 minutes of taxi runs on the ground this weekend. Engine started fine and run fine. But after about 10 to 15 minutes, the engine shut itself down at around 1000 rpm. If I catch it before it stop and "clear it" by adding power to above 1500 rpm, it won't die but in a few minutes I needed to repeat "clearing it" or it would die at low rpm. After restart, the engine would run continuously at as low as 800 rpm. The cylinder head temp was 350 when the engine started to die at low rpm. The ambient temperature was about 65-70. I am thinking vapor lock?

2. During taxi tests, I did a lot of restarts and I only run it to full power for couple minutes. When I was done, I expected the battery was somehow low in charge, but I did not recharge the battery. The next day, the cold engine won't start. The starter turned over with good speed, but the engine won't even cough. After about 4 tries, the starter started to run weak, then I gave up. I wonder if running 100LL doing taxi test runs caused the spark plugs to foul, but I have not open the cowling to check spark plugs? I also am wondering what is the minimum battery voltage for the primary to fire? Maybe the ignition system needs a higher voltage then the starter?

3. During previous flight, the engine died as soon as I turned off the runway at idle rpm. I was able to restart after cranking for a few seconds. Vapor lock?

The fuel system has a gravity feed tank, an on-off ball valve, an in-line filter, a gascolator on firewall then Aerocarb. I run 100LL.

Any advise or suggestion is welcome.

Thanks in advance.

Robin

You definately have a vapor problem. The engine stoppage at taxi, after a flight, is due to the heat soaked engine heating the fuel line and gascolator. I, and others, went through the same thing. A Sonex friend had to do a 180 and land downwind due to "burps" during climb out on one of his first flights. I took the insulation off last fall to do some maintenance and didn't put it back on for the winter. Last week the temp hit 60-65 and I experienced burps during taxi and stoppage after landing. It will run at higher throttle settings because it's moving the fuel fast enough to prevent or absorb the vapor bubbles or because you are moving more air through the cowl. I'm not burning 100 LL but the problem is still the same.
Several things solve the problem. Insulate everything that contains fuel in front of the firewall. The gascolator is difficult to wrap with pipe insulation and I eventually removed it. It's primary purpose in a Sonex is to trap water. The factory position is, if there is any water in the system, it will be at the lowest point, which is the AeroInjector, and the engine will not start. I have owned and flown a Cessna 150 for 25 years and have never found water in the tanks or the gascolator, which is inspected at every annual. At first I didn't insulate the gascolator but on a 90 degree day, after an hour of flying, it happened again, after I got on the ground.
The size of the outlet in the bottom cowl is also important. The outlet in my Onex was increased, an engineering change by the factory, and it helped a lot with the under cowl temperatures.
For insulation we use a foam pipe insulation that has a peel and stick surface and an overlapping joint seal. The smallest size works fine, and if you have to double up, the largest size fits over that. Good luck with keeping you fuel cool.